In the given organic compound, the carbon atoms numbered as $1,2,3,4,5,$ and $6$ are $s p, s p, s p^{3}, s p^{3}, s p^{2},$ and $s p^{2}$ hybridized respectively. Thus, the pair of hybridized orbitals involved in the formation of $C _{2}- C _{3}$ bond is $s p-s p^{3}$